Pasting a link whose text/html label is a shortened form of its URL plants a `[` inside the URL

从 src/paste-markdown-html.ts 的第 83-90 行附近开始，这里是 convertToMarkdown 在纯文本中定位 HTML 标签的位置。复现 issue 中的剪贴板案例，然后添加提议的修复中描述的四个回归测试，并运行现有测试套件。完成的标准是：缩短的 URL 标签能够正确替换且不会破坏 URL，同时其他情况保持当前行为不变。

What happens

When a clipboard carries a URL as text/plain and an anchor labelled with a shortened rendering of that URL as text/html, pasting into a GitHub comment box inserts the Markdown link syntax inside the URL:

text/plain: https://github.com/owner/repo/blob/main/a.js#L7
text/html:  <a href="https://github.com/owner/repo/blob/main/a.js#L7">repo/blob/main/a.js#L7</a>

pasted:     https://github.com/owner/[repo/blob/main/a.js#L7](https://github.com/owner/repo/blob/main/a.js#L7)
expected:   [repo/blob/main/a.js#L7](https://github.com/owner/repo/blob/main/a.js#L7)

It renders as two links, one of them junk (https://github.com/owner/ on its own), and the comment has to be repaired by hand.

Reproduction

Run this in the console of any page with a GitHub comment box, press ⌘C/Ctrl+C, then paste into the box:

document.addEventListener('copy', event => {
  const url = 'https://github.com/owner/repo/blob/main/a.js#L7'
  event.clipboardData.setData('text/plain', url)
  event.clipboardData.setData('text/html', `<a href="${url}">repo/blob/main/a.js#L7</a>`)
  event.preventDefault()
}, {once: true})

Why it happens

convertToMarkdown starts from the plaintext flavour and splices [label](href) over the label at the offset where indexOf finds it:

https://github.com/github/paste-markdown/blob/main/src/paste-markdown-html.ts#L83-L90

That is correct for prose containing a link, where the plaintext is a flattening of the HTML. Here the plaintext is the URL and the label is a substring of it, so the label is found at offset 25 rather than 0 and the splice covers only the tail.

The same corruption happens when prose wraps the URL in punctuation — plaintext (https://github.com/owner/repo/blob/main/a.js#L7) yields (https://github.com/owner/[repo/…#L7](…)).

Who hits it

Any tool that writes a link to the clipboard as "URL as plaintext, shortened label as HTML" — macOS automation scripts (Hammerspoon/Alfred/Keyboard Maestro), clipboard managers, and "copy link" affordances that shorten the visible text. Labels containing a character the URL does not (repo#1234) are unaffected, because indexOf misses and the paste is left alone; every label that is a substring of its own URL is affected.

Proposed fix

#117 — replaces the whole URL when the label is a shortened rendering of it, declines rather than corrupting when the label sits inside a URL that is not its own href, and leaves every other case on the existing path. Four regression tests, all failing before the change.